Washington - Support for the majority Republican party in the United States is sagging as President George W. Bush's popularity continues to slide, according to a poll released on Wednesday.

A plurality of Americans, 48%, said they would prefer the Democrats to control Congress compared to 39% who want the Republicans in power, said the poll commissioned by United States television channel NBC and the Wall Street Journal.

The gap between the two parties was the largest recorded since the NBC/Journal poll started asking the question 11 years ago.

The Republicans hold a majority in both houses of Congress and face mid-term elections next year amid growing public concern over the war in Iraq and high energy prices.

The poll showed Bush's approval ratings dropping to 39%, the lowest of his presidency in the NBC/Journal surveys. Other polls have shown a similar decline with Bush's ratings falling below the 40% threshold in recent weeks.

Anxious about Bush's plunging poll numbers, Republicans in Congress have begun to break ranks and defied the White House on important issues.

The poll also revealed overwhelming opposition to Bush among African-Americans. Only two percent said they approved of his performance as president, the lowest level ever recorded in that category, NBC television reported.

According to the survey, US voters are anxious about rising gas and energy costs, with a majority saying they expected prices to increase further.

Investigations of prominent Republican lawmakers also appear to pose a threat for Bush's party.

Sixty-five percent said charges against Bush ally and former majority leader of the House of Representatives, Tom DeLay, suggested potential illegal activity.

A majority of 57% said an investigation into possible insider trading by the majority leader in the Senate, Bill Frist, indicated possible wrongdoing.

The poll, based on interviews with 807 adults, was conducted between October 8 and October 10 and had a margin of error of 3.4 percentage points.